Cardiac myocytes display two developmentally regulated forms of growth, namely hyperplastic (increased cell number) and hypertrophic growth (increased cell size). At a mechanistic level, the signals that distinguish hypertrophic growth from cell cycle progression are unknown. However, since cell cycle progression is intimately linked to cell mass, there must be factors, which functionally link them. One endogenous factor implicated in the regulation of both hyperplastic and hypertrophic growth in cardiac muscle is the immediate early transcription factor, c-Myc. To explore its role specifically in adult myocardium, we created a novel inducible cardiac-restricted transgenic model whereby we can temporally regulate Myc activity in the heart (MycER). We have demonstrated that Myc alone is sufficient to induce both hypertrophic growth and cause cell cycle reentry, even in adult post-mitotic cardiac myocytes. Our preliminary data suggest that Myc is also necessary for the development of cardiac hypertrophy in vivo since subjecting Myc-null myocardium to hemodynamic stress results in apoptosis and replacement fibrosis. These data raise a number of interesting and testable hypotheses regarding the role of Myc in regulating cardiac growth and survival. We hypothesize that Myc regulates hypertrophic and hyperplastic growth by distinct transcriptional programs and that it is necessary in normal growth to prevent apoptosis. This proposal will attempt to genetically dissect cardiac growth control and specifically Myc's role by directly assessing the importance of Myc in hypertrophic growth by using MycER mice to determine the basis for Myc-mediated hypertrophic growth in adult myocardium (Aim 1), determine the molecular mechanisms that differentiate Myc-induced hyperplastic and hypertrophic growth (Aim 2) and explore the mechanisms underlying the requirement of Myc for a normal hemodynamic response by clarifying the role of Myc in regulating cardiac myocyte apoptosis (Aim 3).